The perforation of petroleum producing wells is realized by lowering into the well various metallic perforating-guns of different lengths, the respective charge carriers of which are charged with shaped charges, connected by joints and fired in a vertical fashion, one after another, resulting in a single unit of joined perforating-guns for the perforation of various zones, in a single lowering.
Each perforating-gun contains a ‘carrier tube’ or charge carrier in which shaped charges, for use in the petroleum industry, of varying geometries are set, Each of the shaped charges detonate conjointly as the charges are in contact with a detonating cord, formed by a sheath containing in its interior a granular explosive with controlled properties, through which the deflagration advances initiated by an electronic detonator, The explosion of the detonating cord detonates the shaped charges sympathetically.
Perforating guns are detonated one at a time beginning from the bottom and continuing in an upward fashion. After each detonation, the gun assembly is repositioned vertically in such a way that the lowest gun that remains active is located at the desired depth of perforation.
The detonator of each gun is activated by an electronic signal sent from the surface to the mouth of the well, by way of an electronic cable, the conductor of which is protected by a steel wire-mesh. Despite having but a single conductive wire to carry any electrical tension to the gun assembly, the charge is passed through diodes and polarized after the detonation of the bottom gun, so as to ensure that the electric current only arrives at the desired gun.
The arming of the guns, i.e., the mechanical assemblage and the electrical connections of the assembly to establish the required firing sequence, is carried out at the wellhead, usually under unfavorable general conditions, making it necessary to provide means to simplify both the arming and the electromechanical connections in order to avoid human error.
Argentine patent application P110102655 discloses a pressure activated electromechanical changeover switch providing the electrical connection. The switch has a conductive dart, which is activated at a predetermined pressure set by a mechanical fuse in the shape of an expendable ring that breaks under stress from the explosion. On receiving the impact produced by the explosion of the shaped charges located in the charge carriers of the lowest active gun in the gun assembly, the dart breaks the mechanical fuse and trips the switch. The dart acts as a switch between contacts that are exposed in the housing of the apparatus and releases the detonator from short-circuit and ground, connecting the detonator of the gun immediately above the fired gun to the circuit. This allows said detonator to later receive the electronic tension, albeit with opposing polarity to the previous gun. This switch, while it is in its initial position, in repose, before moving, allows the electrical charge, positive or negative, to reach the bottom gun and maintains the remaining and unfired portion of the gun assembly short-circuited and grounded.
It is also disclosed in the referred application P110102655 that, in each gun, each charge carrier is limited by two insulating and centralizing end plates: the bottom with a central orifice through which the firing line and the unit ground to a tab located on the outer edge of the end plate pass, placing the unit ground in contact with the outer shell of the gun, which acts as the ground for the entire gun assembly (bottom end plate without retractable contact pin), both cables are tied to a connector; and the top end plate that has a spring-loaded electrical contact pin which function is, on one hand, to assure constant electrical contact with the preceding switch, in spite of variances in lengths that can be produced, and on the other, to prevent failure of the cables that connect the guns due to twisting, as frequently occurs with current practices. The bottom plate without said pin, serves to connect the ground to the gun assembly, and to space and centralize the charge carrier within the gun. Both end plates serve to protect the elements that form the series of guns (top sub, tandem sub, and bottom sub) from fragments produced by the explosion, in such a way that these parts may be used again.
In the referred Argentine patent application P110102655, the top and bottom end plates of each carrier comprise cylindrical pieces with diameters greater than their height, the exterior of which is a cylindrical surface of a diameter that is intermediate between the inner diameter of the hollow cylinder of each gun and the external diameter of the respective carrier tube. Each piece has a means to center the carrier and an annular recess that is aligned with the corresponding end of the carrier.
The existing end plates used in TCP systems (tubing-conveyed perforation) have a fixed insert (the end plates and inserts are currently made of aluminum and steel, and formed by various pieces). In order to be able to use them in the so-called Wireline Systems, the inserts must be removed and the retractable contact pin with the wiring contact screw must be placed in the top of the assembly.